Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1996-05-10
1998-03-24
Cabeca, John W.
Electricity: motive power systems
Induction motor systems
Primary circuit control
318812, 318438, 318808, 318729, H02P 0540
Patent
active
057316821
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL SCOPE
This invention relates to a control unit for motors and a control method which are suitable for use in which induction motors (hereafter referred to as "motor") are operated at the maximum efficiency at all times irrespective of load fluctuations (variations).
BACKGROUND
Operation of a motor by inverter is ordinarily carried out under the condition where V (voltage)/f (frequency) is fixed. If operation frequency is determined, the motor experiences slip where the efficiency is maximum. This value takes a fixed value irrespective of whether load is high or low (light or heavy). In the operation where V/f is fixed, if frequency (f) is determined, since voltage (V) is fixed, slip changes when load changes. Accordingly, the efficiency changes, thus failing to achieve out operation at the maximum efficiency at all times. Explanation will now be briefly given by taking examples of conventional technology of this kind.
As a method for overcoming lowering of efficiency resulting from change of load which has been proposed as the first conventional example, there is an operation method disclosed in the Tokkohei No. 3-41029 (Japanese Patent Publication No. 41029/1991). This operation method is directed to an operation method for induction motors driven by voltage type inverters, wherein for a time period during which the induction motor is accelerated or decelerated, inverter output voltage is regulated so as to carry out operation by V/f constant control to conduct a control so that ratio V/f between a set inverter output voltage and an inverter output frequency is constant (fixed). In this way, when the induction motor is brought into the mode of constant operation, the method comprises the steps of determining output power P.sub.N by performing multiplication of inverter output voltage E.sub.DCN and inverter output current i.sub.DCN. Decreasing the inverter output voltage from output voltage V.sub.0 at the time of beginning of the constant operation to voltage V.sub.1 dropped (lowered) by .DELTA.V, comparing difference .DELTA.P1 between the inverter output power P.sub.1 calculated at this time and the inverter output power P.sub.0 at the time of beginning of the constant operation with constant .DELTA.P.sub.s set in advance, it fixes the inverter output voltage to voltage V.sub.1 if .DELTA.P.sub.1 <.DELTA.P.sub.s. On the other hand, further decreasing, the inverter output voltage has elapsed by .DELTA.V from voltage V.sub.1 after a predetermined time if .DELTA.P.sub.1 .gtoreq..DELTA.P.sub.s, a comparison can be made of an output power decrease value at that time and the constant .DELTA.P.sub.s, and repeating fixing or further decreasing of the inverter output voltage in dependency upon magnitude of the comparison result to operate the induction motor by inverter output voltage.
Further, in the Tokkaihei No. 3-261394 (Japanese Patent Application Laid Open No. 261394/1991) which has been proposed as the second conventional example, there are disclosed a unit and a method adapted for setting, in accordance with load factors, power factors in which power becomes minimum to control voltage so that this power factor is provided. This unit comprises a means for detecting voltage delivered to windings of a motor, means for detecting a current flowing in the winding, power calculation (operation) means connected to the voltage detecting method and the current detecting method, and operative to calculate active (effective) power for at least one cycle from the product of the voltage and the current, and to calculate apparent power for at least one cycle from product of effective value of the voltage and effective value of the current. In addition, the method for setting optimum power factor command value of the motor; the comparison means for calculating measurement power factor from the active power and the apparent power to compare the measurement power factor and the set power factor to generate a power factor comparison signal; and voltage regulation method responsive to the power factor comparis
REFERENCES:
patent: 5272429 (1993-12-01), Lipo et al.
patent: 5420778 (1995-05-01), Yoshino
patent: 5500581 (1996-03-01), Hatanaka et al.
patent: 5521482 (1996-05-01), Lang et al.
Cabeca John W.
Kabushiki Kaisha Yaskawa Denki
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